1. Field of the Invention
The present invention relates to an image formation apparatus which forms an image onto a photosensitive body by using a light source.
2. Related Background Art
In a digital image formation apparatus such as a copying machine, a printer or the like which adopts electrophotographic techniques, light corresponding to an original image is irradiated by a light source such as a laser or the like onto a photosensitive layer on the surface of a photosensitive body uniformly charged by a charger utilizing corona discharge or the like, whereby an electrostatic latent image is formed on the photosensitive body. Recently, the apparatus which uses a semiconductor laser as the light source has been widely used. In a latent image formation process, a laser beam is initially collimated by a lens, and the collimated beam is irradiated onto a rotating polygonal mirror, whereby the laser beam scans the surface of the photosensitive body along a photosensitive body axis direction by the rotation of the polygonal mirror. A thus formed electrostatic latent image is applied with toner by a development unit, whereby the latent image on the photosensitive body is visualized. Then such a toner image is transferred onto a sheet member by a transfer unit. The toner image transferred onto the sheet member is heated, pressed and fixed thereto by a fixing unit. On the photosensitive body posterior to the image transfer, the toner not transferred to the sheet member is eliminated by a cleaning unit, and then the residual electrostatic latent image on the body is also eliminated by light irradiation with a pre-exposure device.
In the digital image formation apparatus which uses such a laser beam, rotational speed of the polygonal mirror has been recently become faster and a multi-beam laser or the like has been recently applied, so that process speed of the apparatus itself has increased. Such increase in the process speed naturally needs an increase in laser rise time speed (i.e., switching response speed). If the laser rise time speed is insufficient, the following disadvantages arise when a binary printer is used for image printing. For example, roughness occurs at a highlight portion, and a fine line or hair stroke is not clearly reproduced.
In order to eliminate such disadvantages, a method has been proposed to always supply a certain quantity of DC current from a laser drive circuit to the laser to improve start of beam emission (i.e., laser ON).
However, such a conventional technique includes the following drawbacks.
As to the potential on the surface of the photosensitive body, a potential difference between the portion corresponding to solid black and the portion corresponding to solid white in a reversal process system (i.e., difference between dark potential and light potential) is generally made as larger as possible so as to improve intent image intensification. For this reason, especially, the apparatus which uses the photosensitive body mainly made of amorphous silicon having relatively low charging capability exposes the portion corresponding to the solid white as little as possible. Namely, in such apparatus, it is desirable to control a current value to be lower than a threshold from which laser beam resonant emission is started, in order to lower a laser beam emission quantity as much as possible. However, there is dispersion in laser characteristics, and there is dispersion also in the identical laser drive circuits controlling currents to be supplied to the laser. Therefore, if the laser current value is determined as one value, a certain apparatus emits the laser beam even at the solid white portion, and, in a certain other apparatus, the laser start speed is delayed. These drawbacks finally cause dispersion in printer characteristics. However, even if the dispersion in every laser characteristic and the dispersion in every laser drive circuit are measured to adjust the current value, it causes increases in cost and manufacturing time.
Further, since operational speed of the image formation apparatus has become faster in recent years, a method in which plural lines are written by using a twin-beam laser capable of simultaneously emitting two laser beams or a laser array consisting of plural laser diodes has been applied to cope with such the fast operational speed.
In the image formation apparatus which uses such plural laser beams, than applied a method disclosed in Japanese Patent Application Laid-Open No. 56-105686 wherein light leaked rearward from each laser diode is detected by using a monitor laser diode, and based on such a detected signal a light quantity of each laser diode is controlled to be equal to others.
Further, in such image formation apparatus, there is also applied a method disclosed in Japanese Patent Application Laid-Open No. 07-319086 in which light quantity distribution of the beam emitted from each laser diode is detected by using a light quantity distribution measurement means, and based on such a detected signal, the exposure quantity of each laser diode is controlled such that the width of a light quantity distribution curve of an effective lower-limit exposure quantity of each laser beam becomes equal to others. Further, in a case where a latent image is formed by each laser beam, the formed latent image is transferred onto a surface of a photosensitive body or a sheet member, and the transferred image is then visualized by using toner. There is also applied a method wherein the density of the visualized image is measured by a density measurement unit, and based on such a measured signal, the exposure quantity of each laser diode is controlled.
However, in the above conventional image formation apparatuses, there were following problems.
That is, in the method as disclosed in Japanese Patent Application Laid-Open No. 56-105686 in which the light leaked rearward from each laser diode is detected by using the monitor laser diode, and based on such detected signal, the light quantity of each laser diode is controlled to be equal to others, and in the method as disclosed in Japanese Patent Application Laid-Open No. 07-319086 wherein the light quantity distribution of the beam emitted from each laser diode is detected by using the light quantity distribution measurement unit, and based on such the detected signal the exposure quantity of each laser diode is controlled such that the width of the light quantity distribution curve of the effective lower-limit exposure quantity of each laser beam becomes equal to others. Differences among respective optical paths extended from emission points of the plural laser beams to the surface of the photosensitive body are not taken into consideration. Therefore, there is some fear that the light quantity on the surface of the photosensitive body can not be correctly controlled.
Further, the light quantity distribution measurement unit and the density measurement unit respectively used in the methods as disclosed in Japanese Patent Application Laid-Open No. 07-319086 cause an increase in cost.